A light-emitting element having a structure in which a light-emitting layer including an organic compound that is a light-emitting substance is provided between a pair of electrodes has attracted attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. Furthermore, a display device including this light-emitting element is superior in contrast, image quality, and has wide viewing angle.
A light-emitting element is said to have a light-emitting mechanism in which an electron and a hole are injected by application of voltage between a pair of electrodes and recombined to form an exciton, so that light is emitted. Electrons and holes each have a spin of ½. It is said that the recombination of electrons and holes allows generation of excitons including 25% spin-0 singlet excitons and 75% spin-1 triplet excitons on the basis of the spin statistics. Note that fluorescence is emitted from a singlet exciton; hence, the theoretical limit of internal quantum efficiency of fluorescence is 25%. The spin-1 triplet excitons constituting 75% of generated excitons relax to the ground state without emitting light in many cases, because they generally do not contribute to light emission. In some cases, triplet-triplet annihilation (TTA) due to a plurality of triplet excitons occurs. TTA is said to be a process in which, by collision between two triplet excitons, energy and a spin angular momentum are exchanged and delivered, generating a singlet exciton.
Development of a phosphorescent light-emitting element having internal quantum efficiency higher than a theoretical value is proposed as well as development of a phosphorescent compound (for example, see Patent Document 1) in order to efficiently use a triplet exciton generated in the light-emitting mechanism. Specifically, a compound with which triplet excitation energy can be converted to light emission (hereinafter referred to as phosphorescent compound) is used in addition to the above-described light-emitting substance, thereby providing phosphorescence due to transfer of energy between states with different multiplicities.